1. Field of the Invention
The present invention relates to a method of protecting an aircraft by signalling against the risks of collision with the terrain in procedures with reduced protection corridor with functions that are specific to these procedures, in order to avoid unwanted warnings emanating from the clearance sensors of the onboard system for automatically signalling risks of collision with the terrain.
2. Description of Related Art
The onboard systems for automatically signalling risks of collision with the terrain, commonly designated by the term “TAWS” (“terrain awareness and warning system”) are stand alone computers or computers integrated with other functions such as the TCAS and WXR in onboard ISS (“integrated surveillance system”) type systems. These TAWS systems fulfil an anticollision surveillance function (“safety net”) with the terrain and their aim is to issue audible warnings on an unintentional approach with the relief, so enabling the crew to react by engaging a vertical resource (called “pull-up”) to avoid the relief before it is too late. To do this, the TAWS systems, independently of the onboard navigation systems, periodically compare the predicted trajectory of the aircraft—including the theoretical trajectory that the aircraft would describe in a “pull-up” type resource—with the terrain being flown over obtained from an onboard digital terrain model (preferably a world model) installed on the computer.
In order to satisfy the increasing demands in terms of air traffic capacity and efficiency, in particular to allow access to certain terrains in steep-sided environments (such as Palm Springs, Calif., or Queenstown, New Zealand), new approach procedures have been introduced (in 2005 in the USA, then elsewhere). These approaches are known by the term RNP, and in particular for some by the name—RNP AR APCH (RNP authorization required approach), previously called RNP SAAAR (required navigation performance—special aircraft aircrew authorization required). They are defined in the ICAO documents referenced PBN No. 9613 and FAA Notice 8260.52 (PBN stands for “performance based navigation”).
The design of these approach procedures is fairly flexible in their creation, because this design is based on the demand for a high level, defined by the standards, of accuracy and integrity of positioning on board the aircraft which will be required to follow this approach. The RNP procedures (predefined trajectories) define a corridor created with a lateral and vertical “framing” determined with a predefined high level of integrity. The RNP AR APCH approaches correspond to approaches with RNPs (lateral distance of the trajectory of the aircraft at the nearest edge of the corridor) typically between 0.3 NM and 0.1 NM (NM standing for nautical miles), that is that the distance for which the RNP AR APCH procedures are defined is equal to twice the value of the RNP between the route to be followed and each side of the procedure so as to protect the route of the aircraft from potential obstacles with a probability of 1.10−5, the lateral deviation tolerance of 0.3 to 0.1 NM relative to the prescribed ideal RNP AR APCH trajectory being given for 95% of cases. Furthermore, these approaches require the aircraft and the crew to be specially provided with special qualifications to be performed.
It will be noted that there are currently several RNP-type and similar procedures (“low RNP”), (“RNP AR APCH”, “RNAV”, etc.). Hereinafter in the text, they will simply be named RNP, but obviously this term here designates any procedure with reduced protection corridor.
On such RNP approaches, the trajectory to be followed is often close to the terrain and along a predefined path, rarely in a straight line. A TAWS system is therefore, even more than for other approaches or situations close to the ground, a precious aid and is even mandatory to perform this type of approach.
However, even more than for other approaches, such a TAWS system ought not to generate unwanted warnings. In practice, such warnings can place the aircraft in extremely difficult situations, because avoidance manoeuvres are not always easy to execute along such approaches.
Now, the design of the current TAWS is such that the protection and warning logics are totally independent of the trajectories followed by the aircraft, because the trajectories to be protected are often less constrained, with or without FMS engaged, than these new approaches, which, it should be remembered, require special authorizations on a per-case basis to be able to be followed.
Consequently, although the TAWS logics are well dimensioned for the general cases, they can be the source of unwanted warnings in RNP approaches, and in particular RNP AR APCH approaches, because the current warning logics are based on a prediction of the future position of the aircraft which cannot be really the position that the aircraft will have a few instants later.
Typically, the current TAWS systems basically comprise at least one or two (even several) clearance sensors, that is, virtual surfaces calculated by the onboard computer and delimiting an exploration volume of the space that is compared to the topography of the corresponding terrain as supplied by an onboard terrain database. The origin of these clearance sensors is located at the predicted position of the aircraft, and consists of an extrapolation of the current trajectory followed by a trajectory representing, taking into account a margin, an avoidance trajectory, and this in a lateral segment to take account of lateral tolerances.
These clearance sensors are determined from parameters of the aircraft such as position, speed, heading or route, altitudes or heights, etc., supplied by the sensors on board this aircraft.
These clearance sensors are used to determine the risks of collision with the terrain by using a database containing at least the elevations of the different areas being flown over.
As illustrated in FIG. 1, the first clearance sensor (1) is determined at most a few seconds in front of the aircraft (2) and is dedicated to the generation of a “warning” type alert (warning of danger involving a resource manoeuvre called “pull-up”). Advantageously, another clearance sensor is determined (3), more in front of the aircraft (typically 30 seconds), and is dedicated to the generation of a “caution” type alert (that is: “attention!”) of lesser importance. As illustrated in FIG. 1, which relates to the case of an initial straight-line trajectory, followed, a little after the instant at which the aircraft has been represented, by a trajectory tending towards the right, as long as this trajectory is straight, these clearance sensors have as their common generation axis the instantaneous route 4 of the aircraft. The problems posed by the curvilinear trajectory (5) are explained hereinbelow with reference to FIGS. 3 to 5 more particularly for the “caution” clearance sensor, even though, to a lesser extent, this problem also concerns the “warning” clearance sensor, provided that its start is not geolocated with the current position of the aircraft.
In certain TAWS systems, the generation axis of the “caution” clearance sensor, like that of the “warning” clearance sensor, is situated in the axis of the instantaneous route of the aircraft (as represented in FIG. 1). In other, more sophisticated TAWS systems, at least one of the clearance sensors, and in particular the “caution” clearance sensor, is situated and oriented along the predicted trajectory of the aircraft, according to a law which is a function of all or part of the instantaneous roll rate. Furthermore, the opening angle (angle of each of the sides of the clearance sensor relative to its generation axis: α1g and α1d for the clearance sensor 1 and α3g and α3d for the clearance sensor 3) of the clearance sensors is more or less great, opening in the direction of the turn, according to a law which is a function of all or part of the instantaneous roll rate, as represented in FIG. 1.
FIGS. 2 (lateral view) and 2A (top view) are diagrams explaining a conventional clearance sensor as considered for the present invention, but without excluding other forms of clearance sensor to which the present invention would be applied. This clearance sensor is created so as to cover the case of a “pull-up” manoeuvre that an aircraft A (represented at an instant T0) must undertake at the latest at a point P1 (that the aircraft will reach after a time period Tr) in order to avoid an obstacle OBST (a hill, for example) situated in front of the aircraft A. A vertical distance between the clearance sensor and the obstacle OBST is defined so that the resource that the aircraft A must begin at the latest at the point P1 is such that the trajectory TR2 of the aircraft beyond P1 will make it possible to retain at least a margin M with the obstacle. In the top view of FIG. 2A, R represents the direction of the route of the aircraft before it begins its obstacle avoidance manoeuvre to the right. Bold lines represent the lateral limits of its “warning” clearance sensor. The left limit forms an angle αG (typically 1.5°) relative to the direction R, and the right limit forms an angle αD, greater than αG, relative to the direction R.
FIG. 3 relates to the case where the aircraft 2 is on the point of beginning a curved trajectory 5 to the right, in the absence of published RNP trajectory. In the case of more sophisticated known TAWS systems, the system then calculates a “warning” clearance sensor 1A, the generation axis of which is oriented along the instantaneous axis of the aircraft at the start point of this clearance sensor and the opening angle α1Ad of which is greater than the angle α1d of FIG. 1, and a “caution” clearance sensor 3A also oriented along the instantaneous axis of the aircraft at the start point of this clearance sensor (notably offcentred relative to the axis of the trajectory 4 and the opening angle α3Ad of which is greater than the angle α3d of FIG. 1).
Even if such a prediction can still be acceptable for the “warning” clearance sensor, it may prove “unorthodox” for the “caution” clearance sensor relative to the reality of the procedure. The predicted positions can thus be notably offset relative to the accepted position tolerance (1 or 2 RNP). Similarly, the exploration segment (the opening angle of the clearance sensor) can notably be offcentred relative to the very probable future orientation of the aircraft. In practice:                As represented in FIG. 4, in the presence of a “turning point” 6 on the trajectory in front of the aircraft 7 predetermined by the published RNP procedure that the aircraft must follow, the “caution” clearance sensor 8 may be situated and remain oriented along the predicted trajectory 9, totally different from the position and the probable future orientation of the aircraft in 30 seconds (RNP trajectory 10 turning to the right from the turning point 6), hence a certain increase in the probability of generating unwanted warnings. In this FIG. 4, as in the subsequent figures, the RNP trajectory has been represented by a “corridor” delimited by broken bold lines and its axis is represented by a continuous bold line. Obviously, the width of this corridor is modulated according to the definition of the procedure.        Similarly, if the predicted trajectory of the aircraft, according to a law which is a function of all or part of the instantaneous roll rate, is greater (as in FIG. 5), or less than the actual rate of turn of the trajectory prescribed by the published RNP procedure, it can prove “unorthodox” relative to the reality of the procedure and thus be notably offset relative to the trajectory to be followed. Taking into account the instantaneous roll of the aircraft 7 to determine the position of the caution clearance sensor 12 can cause this clearance sensor to examine an inappropriate segment, outside the RNP trajectory 11.        
It should be noted that, even if, for such approaches, the aircraft must be qualified to perform RNP approaches and have onboard systems guaranteeing the precision and the integrity of the aircraft position, its TAWS must remain operational to monitor errors of altitude and to intervene in the case of failure to observe the required precision, for example in the case of failure of certain onboard systems.
This therefore means, for the TAWS, having an operation with the fewest unwanted warnings when the aircraft follows the prescribed procedure, and on the other hand fully retaining its nominal warning mode when this procedure is no longer followed.
A terrain avoidance device for aeroplanes is known according to the patent application EP 1 857 781 A2, in which it is simply specified that the “warning” clearance sensor alone is reduced so as to enter into the RNP volume and possibly offcentred to be “centred” on the RNP trajectory, which does not make it possible to adapt the clearance sensor to all the RNP situations and avoid unwanted warnings.